A prospective, controlled study of nutritional status in 100 newly diagnosed paediatric oncology patients is reported. Results showed that although patients' height-for-age and weight-for-height were normal, mid-upper arm circumference (MUAC) and triceps skinfold thickness (TSFT) were significantly less than reference and control values. Moreover, 20% of patients had MUAC below the 5th percentile (P less than 0.001) and 23% had TSFT more than 2 SD below the mean (P less than 0.001). The discrepancy between these assessments is attributed to the effect of large tumour masses masking diminished true body weight. Children with intra-abdominal solid tumours had significantly lower MUAC and TSFT than those with either extra-abdominal solid tumours or leukaemia, and were more likely to be malnourished based on arm anthropometric criteria. Of all subjects, 44% were consuming less than 80% of their recommended daily allowance of energy at diagnosis, compared with 0% of controls (P = 0.001).